1. Field of the Invention
The invention relates to a method and circuit for iris control, and to a camera having such an iris control circuit.
2. Description of the Related Art
Cameras often have interchangeable lenses with an electrically controllable iris. There are two types of iris-controlled lenses: passive iris-controlled lenses, and active iris-controlled lenses. A passive iris-controlled lens only includes coils to realize the iris adaptation. All electronics is situated outside of the lens. An active iris-controlled lens has electronics inside of the lens unit. The iris is commonly controlled by applying a buffered sensor signal to the lens. In response, the lens controls the iris in such a manner, that the amplitude of the sensor signal has a fixed value.
Back-light compensation is commonly realized by decreasing the size of the measurement window used for the exposure control. For example, the measurement window is reduced to a rectangular area or a vertical bar in the middle of the image. For lens control, a same measurement window needs to be used to have the lens control correspond to the exposure control. In some prior art cameras, this is effected by applying the buffered sensor signal to the lens only during the measurement window.
It is, inter alia, an object of the invention to obtain an improved iris control. To this end, a first aspect of the invention provides a method of generating an iris control signal comprising the steps of furnishing an image signal; and subjecting the image signal to a sample-and-hold operation to form the iris control signal. A second aspect of the invention provides a circuit for generating an iris control signal, comprising means for furnishing an image signal; and means for subjecting the image signal to a sample-and-hold operation to obtain the iris control signal. A third aspect of the invention provides a camera comprising a lens having a controllable iris coupled to receive an iris control signal, and the above circuit for generating the iris control signal.
In a method of generating an iris control signal in accordance with a primary aspect of the invention, an image signal is subjected to sample-and-hold operation to obtain the iris control signal.
The invention is based on the recognition that most lenses have problems with control signals which are only available during a given window, instead of being continuously available.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.